I don't see how the 10th Amendment could be any clearer. If the Federal Government is going to declare an action by a state to be illegal or invalid, then it should have the burden placed on it to show where in the Constitution that action taken by the state is forbidden.

Even the military victory of the north and the occupation of the states that had seceded did not resolve the question. If it had, then there would have been no need for the Supreme Court to address it in the case of Texas v. White (1869). The declaration by the Court at that time that the states had never actually left the Union did not make secession illegal in 1860 and 1861. Article I, Section 9 of the Constitution forbids ex post facto laws.